chore(persistent_sessions): tune mnesia parameters for better dump behavior

apps/emqx/etc/emqx_cloud/vm.args

@@ -116,3 +116,7 @@
 
 ## patches dir
 -pa {{ platform_data_dir }}/patches
+
+## Mnesia thresholds
+-mnesia dump_log_write_threshold 5000
+-mnesia dump_log_time_threshold 60000

apps/emqx/etc/emqx_edge/vm.args

@@ -114,3 +114,7 @@
 
 ## patches dir
 -pa {{ platform_data_dir }}/patches
+
+## Mnesia thresholds
+-mnesia dump_log_write_threshold 5000
+-mnesia dump_log_time_threshold 60000